The present invention relates generally to a zoom lens and an imaging system incorporating it, and more particularly to a three units type zoom lens comprises a solid-state imaging device suitable for compact digital cameras and an imaging system that incorporates it.
In recent years, digital cameras and video cameras using solid-stage imaging devices have seen wide use. Such digital cameras and video cameras are now required to have more compact and more multifunctional arrangements. To meet such demands, zoom lenses having much higher image-formation capabilities are in need. In addition, those digital cameras and video cameras are still left much to be desired in fabrication cost while keeping high image quality intact.
For zoom lenses suitable for use with solid-state imaging devices, for instance, the following three patent publications are now available.
Patent Publication 1
JP(A) 2001-318311
Patent Publication 2
JP (A) 2004-61675
Patent Publication 3
JP(A)2003-140041
Patent publication 1 comes up with a three units type zoom lens comprising, in order from its object side, a first lens unit of negative refracting power, a second lens unit of positive refracting power and a third lens unit of positive refracting power, wherein for zooming purposes, the first lens unit and the second lens unit are operable to move from the wide-angle end toward the telephoto end.
Patent publication 2 proposes a three units type zoom lens system comprising, in order from its object side, a first lens unit of negative refracting power, a second lens unit of positive refracting power and a third lens unit of positive refracting power, wherein upon zooming from the wide-angle end to the telephoto end, each lens unit is operable to move.
Patent publication 3 puts forward another three units type zoom lens comprising, in order from its object side, a first lens unit of negative refracting power, a second lens unit of positive refracting power and a third lens unit of positive refracting power, wherein for zooming purposes, the first lens unit and the second lens unit are operable to move from the wide-angle end toward the telephoto end. This zoom lens system is rather improved in that, albeit having a zoom ratio of about 3, it is composed of as small as five subunits or six lenses and has an aperture ratio of at least 1:2.8 at the wide-angle end.
However, a problem with the zoom optical system of patent publication 1 is that it is composed of a bit more lenses or seven lenses, ending up with higher fabrication costs.
Although the zoom lens set forth in patent publication 2 is smaller in the number of lenses, and more improved in optical performance, than that disclosed in patent publication 1, yet the whole length of the lens system from the vertex of the lens surface nearest to the object side to the imaging plane of a solid-state imaging device is relatively long and an axial lens-to-lens space is comparably wide, offering a problem that the whole thickness of a lens barrel upon received at a collapsible lens mount grows large.
The optical system set forth in patent publication 3 is still less than satisfactory in terms of compactness upon housed in place, because the thickness of each lens is large, and moreover its whole length is long relative to its focal length.